1. Field of the Invention
This invention relates to a composition and method for forming a calcium hydroxide slurry for use as an alkaline neutralizing agent.
2. Description of the Prior Art
Alkaline products which are used in adjusting pH in commercial activities, such as the treatment of sewage or wastewater, typically employ sodium hydroxide solutions (commonly referred to as caustic) or suspensions of magnesium hydroxide. Sodium hydroxide is widely available commercially as typically found in a solution having an activity of approximately 50%. As a true solution, 50% caustic is indefinitely stable and has a relatively low viscosity. Magnesium hydroxide is not a solution but rather a suspension of small particles in water. Magnesium hydroxide, however, is not indefinitely stable without agitation, and the particles eventually settle out of suspension forming a sediment layer.
Both caustic and magnesium hydroxide have inherent disadvantages. A 50% strength caustic solution is very dangerous and can result in serious chemical burns if extensive safety precautions are not followed. Further, magnesium hydroxide reacts much more slowly than sodium hydroxide so that an excess amount of magnesium hydroxide must be typically added to produce adequate reaction rates.
Lime has a relatively rapid reaction rate and is fairly safe to handle, making it preferable to use in place of caustic soda or magnesium hydroxide. Further, lime is less costly than caustic or magnesium hydroxide. It has a relatively stable price which fluctuates much less than the price of caustic. Several disadvantages, however, prevent conventional lime slurries from being used where caustic soda solutions would otherwise be used. Because lime slurries are composed of individual calcium hydroxide particles, these calcium hydroxide particles will tend to settle from suspension. Further, the viscosity of conventional lime slurry is high compared to caustic soda. It is difficult to produce a conventional lime slurry with a solids content of greater than 32% without producing such a high viscosity that the material becomes unmanageable. Typically, a viscosity above approximately 2000 cps can cause handling difficulties. Lime slurry can be made less viscous by the addition of gypsum during slaking. Solids content in excess of 40% can be produced in this way. However, the effect of the gypsum is to agglomerate the lime making an abundance of large calcium hydroxide particles. These large lime particles quickly settle from suspension and form a hard sediment that is hard to remove and causes plugging of pipes. In addition, large lime particles react more slowly and display other detrimental characteristics.
To make a lime slurry with a fine particle size, either hydrated lime (Ca(OH).sub.2) or quicklime (CaO) with no gypsum must be used. Hydrated lime requires mixing, powder handling, and dust control equipment that most facilities lack. In addition, neutralizing with hydrated lime is time consuming and can generate a large amount of dust. The other alternative is slaking quicklime without gypsum. This requires slaking equipment and quicklime storage facilities. Slaking is also time and labor intensive and generates oversize "grit" that must be removed and disposed of properly. Hot solutions generated during lime slaking can also pose a burn hazard.
In forming high solids content lime slurries with low viscosities, large amounts of dispersing agents are used with hydrated lime. High amounts of dispersants are undesirable in many applications. For instance, neutralized waste water is commonly treated in an aeration basin. Excess dispersant can cause foam to accumulate on the aerated waste resulting in significant processing difficulties. Also, polymeric dispersants add to the total organic content of material treated which can be undesirable. Further, high dispersant amounts would add considerably to the cost of producing lime slurries for use as a caustic replacement. It is therefore necessary to reduce or minimize the amount of dispersants, especially the polymeric dispersants in formulating a low viscosity of lime slurries for a caustic replacement. As much as 5% dispersant based upon the dry weight of lime has been used in order to make a pumpable slurry having a solids content of lime between 45 to 50%.
It is therefore an object of this invention to produce a fine grain material that can be used as a caustic replacement that does not settle readily from suspension.
It is another object of this invention to provide a caustic replacement which displays a relatively low viscosity at high solids content and that has a substantially identical neutralizing capacity as a solution containing an equivalent amount of caustic.
Further, it is an object of this invention to produce a lime slurry having a high solids content of more than about 39% by weight and a viscosity of less than 2000 cps, and preferably less than 1000 cps.
It is another object of this invention to provide a lime slurry that remains stable for several days without settling into a hard packed sediment that cannot be easily resuspended.
It is yet another object of this invention to provide a lime slurry that is of low viscosity and high solids content with little dispersing agent.